In recent years, along with enhancement of the operation rate of personal computers, and popularization of network infrastructure, as well as an increase in capacity of data storage and a decrease in its cost, occasions have increasingly occurred in which pieces of information such as documents and images, which have been provided in the form of paper printed matter, are received as simpler electronic information and viewed as received electronic information.
As viewing means for such electronic information, mainly employed are those of light emitting types such as conventional liquid crystal displays and CRTs or recent organic electroluminescence displays. Specifically, when electronic information includes document information, it is required to watch any of the above viewing means for a relatively long period. However, it is hardly stated that the above viewing means are human friendly. It is common knowledge that light emitting type displays result in problems such as eye fatigue due to flicker, inconvenient portability, limitations in reading posture, necessity to look at still images, or an increase in power consumption.
As means to overcome the above drawbacks, are known reflection type displays (having memory function) which utilize outside light and consume no power to maintain images. However, it is difficult to state that due to the following reasons, they exhibit sufficient performance.
Namely, a system employing polarizing plates, such as a reflection type liquid crystal, results in a problem for a white display due to a low reflectance of approximately 40%. In addition, it is difficulty to state that most methods to produce structuring members are simple and easy. Further, polymer dispersion type liquid crystals require high voltage and the contrast of the resulting images is insufficient due to utilizing the difference in refractive indices between organic compounds. Still further, polymer network type liquid crystals result in problems such as application of high voltage and requirement of complicated TFT circuitry to enhance memory capability. Yet further, display elements employing electrophoresis require high voltage of at least 10 V and tend to suffer insufficient durability due to aggregation of electrophoretic particles. Further, electrochromic display elements, though being drivable at a low voltage of at most 3 V, result in insufficient color quality of black and common colors (namely yellow, magenta, cyan, blue, and red) and tend to result in problems such that, in order to secure memory capability, the display cell requires a complicated film structure such as vapor deposition film.
As a display system, which overcomes the drawbacks of each of the above systems, an electrodeposition (hereinafter referred to as ED) system has been known which utilizes dissolution and deposition of metals or salts thereof. ED systems exhibit advantages such as drivability at a low voltage of at most 3 V, a simple cell structure, excellent black and white contrast, or excellent black quality, for which various methods have been disclosed (refer, for example, to Patent Documents 1-3).
The inventors of the present invention conducted detailed investigation of technologies disclosed in each of the above Patent Documents, and came upon the following problems. In conventional technologies, when white pigments were dispersed in an electrolyte, the resulting dispersion stability of the white pigments was insufficient, whereby white display reflectance fluctuated over an elapse of time. Alternatively, when non-woven fabric or filter paper is employed as a white scattering member, difference in the refractive index from organic materials in the electrolyte is small. As a result, problems were found in which white display reflectance was not sufficient, leading to further investigation. As a result, it was found that these problems were solved by arranging porous white scattering materials between counter electrodes. However, it was found that non-uniform thickness of the white scattering layer resulted, depending on the preparation method of the relatively thick white scattering layer, whereby image mottle is formed.
Patent Document 1: U.S. Pat. No. 4,240,716
Patent Document 2: Japanese Patent No. 3428603
Patent Document 3: Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 2003-241227